In many electronic applications, electrical resonators are required. For example, in many wireless communications devices, radio frequency (rf) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Film Bulk Acoustic Resonator (FBAR). The FBAR has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators.
Desirably, the bulk acoustic resonator excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., z-direction) of the piezoelectric layer.
Unfortunately, besides the desired TE modes there are lateral modes, known as Rayleigh-Lamb modes, generated in the acoustic stack as well. The Rayleigh-Lamb modes are mechanical waves having k-vectors that are perpendicular to the direction of TE modes, the desired modes of operation. These lateral modes travel in the areal dimensions (x, y directions of the present example) of the piezoelectric material.
Among other adverse affects, lateral modes deleteriously impact the passband response of an FBAR filter. In particular, the Rayleigh-Lamb modes establish discrete modes at resonance frequencies as determined by the lateral dimensions of the resonator. The result is that in measuring the insertion loss of the passband portion of the filter, one sees ‘ripples’ or ‘dings’ in the pass band where energy incident on the filter is not passed thru to the output, but “sucked out” and converted as heat or other mechanical energy.
While attempts have been made to improve the insertion loss as well as the quality (Q) factor of known FBARs, certain drawbacks remain. In addition, because wafer ‘real estate’ remains at a premium, there is a need to maximize the use of each unit area of wafer, to within the limits that electrostatic discharge will allow.
What are needed, therefore, are an acoustic resonator structure and an electrical filter that overcomes at least the shortcomings of known described above.